The making of cellulosic fiber sheets, particularly paper and paperboard, includes the following: 1) producing an aqueous slurry of cellulosic fiber; which may also contain inorganic mineral extenders or pigments; 2) depositing this slurry on a moving papermaking wire or fabric; and 3) forming a sheet from the solid components of the slurry by draining the water.
The foregoing is followed by pressing and drying the sheet to further remove water. Organic and inorganic chemicals are often added to the slurry prior to the sheet-forming step to make the papermaking method less costly, more rapid, and/or to attain specific properties in the final paper product.
The paper industry continuously strives to improve paper quality, increase productivity, and reduce manufacturing costs. Chemicals are often added to the fibrous slurry before it reaches the papermaking wire or fabric, to improve the method drainage/dewatering and solids retention; these chemicals are called retention and/or drainage aids.
As to drainage/dewatering improvement, drainage or dewatering of the fibrous slurry on the papermaking wire or fabric is often the limiting step in achieving faster method speeds. Improved dewatering can also result in a drier sheet in the press and dryer sections, resulting in reduced steam consumption. In addition, this is the stage in the papermaking method that determines many sheet final properties.
With respect to solids retention, papermaking retention aids are used to increase the retention of fine furnish solids in the web during the turbulent method of draining and forming the paper web. Without adequate retention of the fine solids, they are either lost to the mill effluent or accumulate to high levels in the recirculating white water loop, potentially causing deposit buildup. Additionally, insufficient retention increases the papermakers' cost due to loss of additives intended to be adsorbed on the fiber to provide the respective paper opacity, strength, or sizing properties.
High molecular weight (MW) water-soluble polymers with either cationic or anionic charge have traditionally been used as retention and drainage aids. Recent development of inorganic microparticles, known as retention and drainage aids, in combination with high MW water-soluble polymers, have shown superior retention and drainage efficacy compared to conventional high MW water-soluble polymers. U.S. Pat. Nos. 4,294,885 and 4,388,150 teach the use of starch polymers with colloidal silica. U.S. Pat. No. 4,753,710 teaches flocculating the pulp furnish with a high MW cationic flocculant, inducing shear to the flocculated furnish, and then introducing bentonite clay to the furnish. U.S. Pat. Nos. 5,274,055 and 5,167,766 disclose using chemically cross-linked organic microparticles or micropolymers as retention and drainage aids in the papermaking process.
Copolymers are also used to control deposition of contaminants or organic deposits in papermaking systems. Organic deposits is a term used to described tacky, water insoluble materials in the papermaking system that are detrimental to the production of paper. Such materials derived from trees during the pulping and papermaking process are termed pitch or wood pitch, while the term stickies is used to describe contaminants that are derived from adhesives introduced into the papermaking process as a contaminant of recycled fiber. One strategy for eliminating these materials is to agglomerate the organic deposits into larger, non-tacky particles that can be removed from the papermaking stock or incorporated into the sheet without causing deposits in the papermaking system of defects in the sheet. Chemicals that are able to interact with organic deposits and mitigate their negative impact include surfactants and polymers. The polymers can be ionic or nonionic, and includes materials used as flocculants, coagulants and dispersants.
The efficacy of the polymers or copolymers used will vary depending upon the type of monomers from which they are composed, the arrangement of the monomers in the polymer matrix, the molecular weight of the synthesized molecule, and the method of preparation. It is the latter characteristic that is a focus of the present invention.
Specifically, it has been unexpectedly discovered that water-soluble anionic copolymers when prepared under certain conditions exhibit unique physical characteristics. Additionally, said copolymers provide unanticipated activity in certain applications including papermaking applications such as retention and drainage aids and contaminant control aids. Although the synthesis methods employed are generally known to those skilled in the art, there is no prior art suggesting that the unique physical characteristics and unanticipated activity observed would result.